External Anal Sphincter

The external anal sphincter (or sphincter ani externus) is a crucial structure in the human body, playing a significant role in the control of defecation. It is an oval tube of skeletal muscle fibers and is part of the anal sphincter complex which includes the internal anal sphincter as well.

Structure and Location

The external anal sphincter is situated at the distal end of the anal canal and is adherent to the skin surrounding the anus. It encircles the terminal part of the anal canal and is under voluntary control. This contrasts with the internal anal sphincter, which is composed of smooth muscle fibers and is under involuntary control.

Layers

The external anal sphincter can be divided into three main layers:

Subcutaneous: The layer just beneath the skin.
Superficial: The central portion.
Deep: The innermost layer, adjacent to the puborectalis muscle, part of the pelvic floor muscles.

Function

The primary function of the external anal sphincter is to maintain continence and control the passage of feces. It works in tandem with the internal anal sphincter to regulate bowel movements. While the internal sphincter provides a baseline level of contraction to maintain continence, the external sphincter can be consciously contracted to prevent the passage of feces, providing additional control.

Reflexes

One notable reflex involving the external anal sphincter is the anal wink. This reflexive contraction occurs when the perineal skin around the anus is stroked. It is an important clinical sign used to assess the integrity of the lower spinal cord.

Clinical Significance

Several medical conditions may involve the external anal sphincter:

Anal fistula: An abnormal connection between the anal canal and the skin near the anus, often originating from the anal glands located between the internal and external sphincters.
Anismus: A condition where there is a paradoxical contraction of the external anal sphincter, making defecation difficult and painful.
Anal incontinence: The inability to control bowel movements due to damage or dysfunction of the external anal sphincter or its neural control mechanisms.

Interaction with Other Systems

The external anal sphincter interacts with various other systems in the body:

The nervous system innervates the external anal sphincter via the inferior rectal nerves, branches of the pudendal nerve.
The circulatory system supplies the region with blood through the inferior rectal artery.
The digestive system relies on the coordinated function of both anal sphincters to control the expulsion of waste.